Absolute vs Relative Humidity
Relative humidity and absolute humidity are two important topics discussed under psychrometrics. These theories are highly important in fields such as meteorology, chemical and process engineering and many more. This article will try to compare these two concepts and discuss the differences and similarities between them.
Absolute humidity is an important factor when it comes to the study of psychrometrics. Psychrometrics is the study of gas – vapor systems. In thermodynamics, the absolute humidity is defined as the mass of water vapor per unit volume of moist air. This can take values ranging from zero to the saturated water vapor density. The saturated water vapor density depends on the pressure of the gas; therefore, the maximum mass of vapor per unit volume also depends on the air pressure. As the pressure, and, therefore, the temperature, affects the absolute humidity, it is inconvenient to use this as an engineering quantity. This is because most of the engineering systems also have variable temperatures and pressures. Therefore, a new definition for the absolute humidity is given. The new definition says absolute humidity is the mass of the water vapor in a volume divided by the mass of dry air in the said volume. This definition is much convenient when dealing with pressure changes. However, to avoid confusion, the first definition is renamed as the volumetric humidity.
Relative humidity is important when the real effect of the humidity is concerned. To understand the concept of relative humidity, there are two concepts that are needed to be addressed first. First one is the partial pressure. Imagine a gaseous system where there are A1 molecules of gas G1 generating pressure P1, and A2 molecules of gas G2 generating pressure P2. The partial pressure of G1 in the mixture is P1/ (P1+P2). For an ideal gas, this is also equal to A1/ (A1+A2). The second concept that has to be understood is the saturated vapor pressure. The vapor pressure is the pressure created by vapor in equilibrium, in a system. Now let us assume that there is still liquid water (however infinitesimal) in a closed system. This means the system is saturated with water vapor. If the temperature of the system is decreased, the system will surely remain saturated, but if it is increased, the result would have to be re calculated. Now let us see the definition of relative humidity. Relative humidity is defined as the percentage of the partial pressure of the vapor divided by the saturated vapor pressure at the given temperature. This is in the form of a percentage. This is a useful quantity in conveying the real feeling of humidity. If the relative humidity is high, we feel sweaty, if it is low, we feel dehydrated. An air conditioned room is a good example of a low relative humid environment. A beach on a hot day is a high relative humid area.
What is the difference between Absolute Humidity and Relative Humidity?
• Absolute humidity is a fraction while the relative humidity is a percentage.
• Absolute humidity cannot give any measure of the real condition since it is independent of the temperature.
• The relative humidity gives a good view of the condition since the saturated pressure depends on the temperature.